Constraining diagenesis within shallow water carbonate environments: Insights from clumped and sulfur isotopes

Abstract

The geochemical records of shallow-water carbonate sediments can be influenced by both meteoric and marine diagenesis, where the contribution from each can be difficult to constrain using traditional δ18O and δ13C values. Here sediment cores from Enewetak Atoll have been utilized as a case study, in which clumped isotope-derived temperatures and calculated δ18Ofluid values are combined with the δ34S values of the carbonate-associated sulfate, to differentiate the types of diagenetic alteration which have influenced the geochemical record, thus providing further insights into the extent and timing of carbonate alteration. While sediments at Enewetak Atoll have undergone repeated periods of subaerial exposure, calculated δ18Ofluid values indicate that the amount of meteoric alteration is reliant on the availability of meteoric fluids, as the presence of exposure surfaces alone does not imply that extensive diagenetic alteration has occurred. Additionally, in the deepest zone of the Enewetak cores, it could be determined that the sediments were initially exposed to meteoric fluids, resulting in partial alteration, and later affected by burial and circulation of deep ocean water, resulting in recrystallization within the section, uniformly overprinting the temperature and δ18Ofluid values. The results of this study indicate that by utilizing systematics such as δ34SCAS, ∆47, and δ18Ofluid values, in addition to the more traditional proxies, a detailed understanding of the diagenetic processes which have altered shallow water sediments can be better constrained, allowing for more accurate interpretations of ancient sedimentary records.